Why do small datasets require more samples, while big datasets require fewer samples in negative sampling?
He likely found this to a be a best practice to avoid over fitting, with a small data set if you only use small and easy to learn (less words -> less degrees of freedom) sequences then you open your model to the risk of over fitting that data set where as on a large data set that has alot more total information you can train on small sequences without being at risk of over fitting because although the smaller sequences will be easier to learn the variance of sequences will be much higher.